The Photo Texture Quality setting is an arbitrary integer number that defines the relative quality of the photograph for texture extraction or auto-coloring. The photograph with the highest quality setting is used first for photo-texture extraction when a Surface appears on more than one photograph. If one or more of the photographs have the same quality value (which is the default) then the photograph with the lowest number is chosen first.
The Photo Quality is a positive or negative integer value. It is only the relationship between the photos' individual qualities that makes a difference. A photo with a higher quality gets chosen first as a texture source if the face being textured appears on that photo.
For example, if one face/Surface appeared on photo 1, 3 and 4 and the photo qualities where set to:
photo 1: quality -1
photo 3: quality 0 (which is the default setting)
photo 4: quality 2
the face would be textured from photo 4 as the search order is 4,3,1.
When the photo quality is the same on two photos, the photo with the lower photo number gets used. So for these five photos:
photo 1: quality -2
photo 3: quality 0
photo 4: quality 2
photo 5: quality 0
photo 7: quality 1
the search order for choosing a texture source would be: 4,7,3,5,1.
You can override the assignment of photo to textured face by using Materials defined as PhotoTexture from photo N. See the Materials Dialog for more detail.